In an LTE (Long Term Evolution) network, a user equipment device (UE) has its uplink synchronized with respect to time. Therefore, a UE which has not yet obtained or has lost its uplink time synchronization must initiate the Random Access (RA) procedure to obtain uplink time alignment. After that, the eNodeB might grant the UE transmission resources.
The LTE specification 3GPP TS 36.321 version 9.0.0 (“Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification), section 5.1 and the LTE specification 3GPP TS 36.300 version 8.9.0 (“Evolved Universal Terrestrial Radio Access (E-UTRA); Evolved Universal Terrestrial Radio Access Network (E-UTRAN), section 10.1.5 define the Random Access (RA) procedure.
To summarise this procedure, a Physical Random Access Channel (PRACH) is provided for the UE to request access to the network. A UE performing RA randomly picks a preamble out of a pool and transmits it. The preamble represents a random UE ID which can be used by the eNodeB when granting the UE access to the network. The eNodeB receiver can resolve RA attempts performed with different preambles and send a response message to each UE using the corresponding random UE IDs. The UE then sends a scheduled transmission.
However, because multiple UEs can request access at the same time, collisions may occur between requesting UEs using the same preamble, and each UE will attempt to send a scheduled transmission. A contention resolution scheme has therefore been implemented to separate the UE transmissions.
However, if multiple UEs simultaneously use the same preamble a collision occurs and some or all of the RA attempts will fail, because the eNodeB cannot distinguish between the multiple users with the same random UE ID.
Thus, collisions between transmissions in the contention-based RA procedure may lead to increased delay and failed access attempts. This limits the RA capacity and/or makes it costly (in terms of resources) to handle high RA intensities. Such limitation is particularly disadvantageous when uplink transmission (necessitating RA) is needed for proper traffic prioritization/handling. This becomes critical in high load and emergency scenarios with many UEs trying to acquire transmission resources.